1. Field of the Invention
The invention relates generally to the field of electromagnetic induction well logging instruments and methods. More particularly, the invention relates to methods for correcting multiaxial induction measurements for “sonde error” effects, and for using the sonde error corrected measurements for more accurately determining dip of conductive rock formations.
2. Background Art
Electromagnetic induction well logging has as one particular purpose the determination of electrical resistivity of rock formations. Electrical resistivity is related to parameters of interest of such formations, including fractional volume of pore space of the formation and the fluid content of the pore spaces. Generally, electromagnetic induction well logging includes moving an instrument along a wellbore drilled through rock formations. The instrument includes one or more transmitter antennas (typically in the form of wire coils) and one or more receiver antennas (also typically in the form of wire coils). Alternating current is passed through the transmitter(s) and signals are detected from the receiver(s) related to voltages induced in the receivers by electromagnetic induction. Characteristics of the induced voltages, for example, amplitude and phase with respect to the transmitter current, are related to the electrical resistivity (conductivity) of the rock formations. Typical induction logging instruments include a plurality of transmitters and receivers spaced apart from each other at selected distances along the length of the instrument so that characteristics of the rock formations may be investigated at a plurality of lateral distances (“depths of investigation”) from the longitudinal axis of the wellbore.
Electromagnetic induction instruments and methods of interpreting the measurements made therefrom include a device used to provide services under the trademark RT SCANNER, which is a trademark of the assignee of the present invention. The foregoing instrument includes a plurality of “multiaxial” antennas, meaning that the antennas each have dipole moments oriented along a plurality of different axes. Each of the multiaxial antennas in the foregoing instrument has a wire coil arranged so that its magnetic dipole moment is along the longitudinal axis of the instrument, and two additional, substantially collocated wire coils arranged so that their dipole moments are substantially perpendicular to the axis of the instrument, and substantially perpendicular to each other. Such antennas may be referred to as “triaxial” antennas. One of the triaxial antennas is used as the transmitter, and a plurality of triaxial coils used as receiver antennas are spaced along the instrument at selected longitudinal distances from the transmitter. The particular arrangement of antennas in any example is not intended to limit the scope of the present invention.
An important purpose for the foregoing induction well instrument is to be able to determine resistivity of rock formations both parallel to the direction of layers of the rock formation (“bedding planes”) and in directions perpendicular to the bedding planes. It is known in the art that certain rock formations consist of a plurality of layers of porous, permeable rock interleaved with layers of substantially impermeable rock including substantial volume of clay minerals. Such formations, referred to as “laminated” formations, have been known to be productive of hydrocarbons and have quite different resistivity parallel to the bedding planes as contrasted with perpendicular to the bedding planes.
An important part of interpreting measurements from the foregoing type of instrument is to correct the measurements for the effects of the wellbore (which occupies some of the volume of investigation of the various receivers) and for the effects of formations having bedding planes disposed at angles other than perpendicular to the axis of the wellbore. Methods known in the art for determining resistivity of such formations using multiaxial electromagnetic induction measurements are described, for example, in U.S. Patent Application Publication No. 2010/0082255 filed by Davydycheva et al, the underlying patent application for which is assigned to the assignee of the present invention.
In some cases, electrically conductive formations may be disposed within or proximate to electrically substantially non-conductive formations. In such cases, depending on the thickness of the conductive formation, it may or may not be possible to determine the exact value of electrical conductivity (resistivity) using induction type instruments. It has been determined, however that such electrically conductive formations so disposed are still susceptible to determination of their geodetic orientation (dip) because the induction instrument is sensitive to their presence. Methods known in the art, such as described in the '225 publication above may be used to determine the orientations of such formations. Because of the low conductivity of the surrounding formations, however, the dip so determined is subject to accuracy limitations as a result of the relatively low accuracy of the underlying induction measurements in substantially non-conductive formations. One technique to improve accuracy is to correct the response of the instrument for “sonde error”, which is a non-zero conductivity output of the instrument when the instrument is disposed in a substantially zero conductivity environment. See, for example, U.S. Pat. No. 7,027,923 issued to Barber et al.
There exists a need for improved methods to determine conductive formation dip wherein there are substantially non-conductive formations present in a wellbore.